Title: When Mount St. Helens erupted, trapped gases caused the north side of the mountain to explode. Volcanic ash was ejected high into the atmosphere.
1- When Mount St. Helens erupted, trapped gases
caused the north side of the mountain to explode.
Volcanic ash was ejected high into the atmosphere.
2- A volcano is a mountain that forms when magma
reaches the surface. - Volcanoes can result from several different
geological processes and can take a variety of
forms.
3Formation of a Volcano
4Formation of a Volcano
- The process that leads to a volcanic eruption
begins deep inside Earth. - Magma rises because it is less dense than the
solid rock around and above it.
5Formation of a Volcano
- How a Volcano Erupts
- Magma is under pressure and contains dissolved
gases, including carbon dioxide and water vapor. - Lower pressure near the surface allows the gases
in magma to expand rapidly. - An eruption occurs when the gases bubble out
through a crack in the crust, propelling magma to
the surface.
6Formation of a Volcano
- Structure of a Volcano
- Before an eruption, magma often collects in a
pocket called a magma chamber. - Magma slowly accumulates in the magma chamber
until enough pressure builds up to start an
eruption. - Then, magma rises to the surface in a narrow,
vertical channel called a pipe.
7Formation of a Volcano
- An opening in the ground where magma escapes to
the surface is called a vent. - Often there is one central vent at the top of a
volcano. Sometimes there are other vents that
open along a volcanos side. - At the top of the central vent in most volcanoes
is a bowl-shaped pit called a crater.
8Formation of a Volcano
- After an eruption, a volcanos magma chamber and
main vent may empty of magma, creating a hollow
shell. - If this shell collapses inward, it creates a huge
depression, called a caldera, at the top of the
volcano.
9Formation of a Volcano
- When a volcanic mountain erupts, magma under
pressure is forced upward from the magma chamber.
Magma flows onto the surface as lava.
Crater
Vent
Magma chamber
Pipe
Lava
10Quiet and Explosive Eruptions
11Quiet and Explosive Eruptions
- Magma can vary in viscosity, the resistance to
flow. - Magma with high viscosity is thick and resists
flowing. Magma with low viscosity is thin and
flows easily.
12Quiet and Explosive Eruptions
- There are three main factors that determine the
viscosity of magma temperature, water content,
and silica content. - Higher temperatures lower the viscosity of magma,
so it flows more easily. - Water in magma helps it flow more easily.
- Magma that is high in silica has high viscosity.
13Quiet and Explosive Eruptions
- Quiet Eruptions
- Volcanoes that have very hot, low-silica magma
generally erupt quietly. - In a quiet eruption, lava erupts in a stream of
low-viscosity lava, called a lava flow. - Lava flows from a quiet eruption can travel for
great distances.
14Quiet and Explosive Eruptions
- Quiet eruptions produce two different kinds of
lava chunks called aa, or smooth coils called
pahoehoe.
pahoehoe
aa
15Quiet and Explosive Eruptions
- Mt. Kilauea in Hawaii erupts quietly, producing
low-viscosity lava flows.
16Quiet and Explosive Eruptions
- Explosive Eruptions
- High-silica magma produces explosive eruptions.
- Thick magma can clog a volcanic pipe, causing
enormous pressure to build up. - When the volcano finally explodes, lava and hot
gases are hurled outward.
17Location and Types of Volcanoes
18Location and Types of Volcanoes
- Volcanoes often form along a converging plate
boundary where an oceanic plate is subducted into
the mantle. - As it sinks through the mantle, the plate causes
melting. - Magma forms and rises to the surface.
19Location and Types of Volcanoes
- Volcanoes also form along a diverging plate
boundary where magma rises to fill the gap
between two separating plates. - Some volcanoes occur at hot spots, regions where
hot rock extends from deep within the mantle to
the surface.
20Location and Types of Volcanoes
- Except for hotspot volcanoes, most of the worlds
volcanoes form near plate boundaries.
21Location and Types of Volcanoes
22Location and Types of Volcanoes
- Different types of volcanic eruptions produce
different types of volcanoes. - Each type is named for its shape or interior
structure.
23Location and Types of Volcanoes
- A quiet eruption of low-viscosity lava produces a
wide, flat volcano called a shield volcano. - If an eruption is entirely ash and cinders, the
result will be a small, steep-sided volcano
called a cinder cone. - A volcano that forms from explosive eruptions
that produce a combination of lava and ash is
called a composite volcano.
24Other Igneous Features
25Other Igneous Features
- Sometimes magma does not reach the surface, but
cools and hardens in the crust. - This magma forms intrusive igneous rock that may
eventually be forced upward and exposed at
Earths surface. - Lava plateaus are features formed of extrusive
igneous rock.
26Other Igneous Features
- A batholith is the largest type of intrusive
igneous rock mass. - Magma sometimes squeezes into a crack between
layers of rock. - If the crack is parallel to existing rock layers,
the magma hardens into a structure called a sill. - If the crack cuts across rock layers, the
hardened magma forms a dike. - When magma hardens in a volcanos pipe, a
structure called a volcanic neck may form.
27Other Igneous Features
- Ship Rock in New Mexico is a volcanic neck. It
formed when the soft rock around a volcanos pipe
wore away, revealing hard, igneous rock. The long
ridge extending from the volcanic neck is a dike.
28Assessment Questions
- An opening in the ground where magma escapes to
the surface is called a - vent.
- crater.
- pipe.
- magma chamber.
29Assessment Questions
- An opening in the ground where magma escapes to
the surface is called a - vent.
- crater.
- pipe.
- magma chamber.ANS A
30Assessment Questions
- A quiet eruption occurs when a volcano is filled
with - very hot, low-silica magma.
- high-silica magma.
- cinders and lava bombs.
- water
31Assessment Questions
- A quiet eruption occurs when a volcano is filled
with - very hot, low-silica magma.
- high-silica magma.
- cinders and lava bombs.
- water.ANS A
32Assessment Questions
- Short, isolated chains of volcanoes, such as the
Hawaiian Islands, occur at - divergent boundaries.
- convergent boundaries.
- hot spots.
- rift valleys.
33Assessment Questions
- Short, isolated chains of volcanoes, such as the
Hawaiian Islands, occur at - divergent boundaries.
- convergent boundaries.
- hot spots.
- rift valleys.ANS C
34Assessment Questions
- What type of structure forms when magma hardens
in a volcanos pipe? - a batholith
- a sill
- a dike
- a volcanic neck
35Assessment Questions
- What type of structure forms when magma hardens
in a volcanos pipe? - a batholith
- a sill
- a dike
- a volcanic neckANS D